Phased Array UT of Dovetails

Table of Contents

Steam Turbine Rotor Dovetail IGSCC

Phased Array Ultrasonic Testing (PAUT) Dovetail Scanner

PAUT Scanning Parameters

PAUT Sector Scan Showing IGSCC Flaw Signals

Steam Turbine Rotor Dovetail IGSCC

The dovetails of a steam turbine rotor may be subject to damage by creep, thermal fatigue, stress corrosion cracking (SCC), and steam erosion depending on steam conditions. The region of greatest concern in a low pressure (LP) rotor is at the load-bearing hooks of the dovetails, where the highest thermo-mechanical stresses exist during service. The LP rotor last stage blading and the next to last stage blading are normally the stages that will have the operating conditions necessary to generate this type of flaws. During operation, pitting and corrosion can occur in the dovetails depending on material susceptibility to corrosion, service stress, and steam quality. Pit formation is considered part of the initiation stage of a SCC attack (see image1). Once these pits start to align, linear flaws also known as intergranular stress corrosion cracking (IGSCC) will initiate and propagate eventually leading to dovetail instability (see image 2 & 3).

Image 1

Image 2

Image 3

Image 4

Cross section of dovetail micrograph showing intergranular stress corrosion cracking (IGSCC) located under each shoulder where stress is the largest. Cracking is more severe on the left side of the photo which corresponds to the steam inlet side (see image 4).

Image 5

Phased Array Ultrasonic Testing (PAUT) Dovetail Scanner

The ultrasonic scanning of the dovetails is performed by first calculating the ultrasonic refracted shear wave beam angle needed to examine the different hook configurations of each stage. To perform these calculations, a cross section drawing of each stage or an actual blade removed from the corresponding stage is used to generate a drawing. The range of refracted angles commonly used for the examination will cover from 30 to 80 degrees. Using these parameters, the scans are performed from the inlet and exhaust faces of each of the stages of the rotors. The R&A PAUT dovetail scanner uses the SONATEST VEO (see image 4) or the HARFANG X-32 (see image 5) PAUT instrument to perform the scanning of the dovetails. Single crystal conventional ultrasonic transducers are also used to verify and assist in detection and sizing of indications.

Image 4

Image 5

PAUT Scanning Parameters

During the scanning of the dovetails, the R&A PAUT scanner is set on a tripod and the rotor is set on power rollers or a lathe or in-situ. The rotor is then rotated at speeds of 1 rpm or less during the scanning. The scanning parameters are shown on the 3D model of a three-hook dovetail (see image 6) showing the PAUT beam tracing and interacting with the geometric features of the dovetail and the expected areas where IGSCC may be present. For the type of dovetails shown below, two scans are performed i.e. from the inlet side to scan the exhaust side and vice versa.

Image 6

PAUT Sector Scan Showing IGSCC Flaw Signals

Below is shown a sector scan that corresponds to one of the scans performed on a flawed dovetail. The PAUT Sector scan shows the cursor extractor at 34.0° (solid line in Sector scan). The Sector scan was obtained during a PAUT examination of a LPA rotor Stage L-1 Gen End inlet side. The dotted outline depicts the dovetail outline. The black arrows show the indications on all three (3) hooks. The red arrow shows the “missing” geometry of hook #1.